The electrolysis of an alkaline brine, carried out with a view of obtaining alkali metal chlorates, leads to the formation of a gaseous mixture containing:
(a) hydrogen formed at the cathodes of the electrolytic cells; and
(b) oxygen, chlorine and possibly carbon dioxide, formed during parasitic reactions which cause a decrease in the Faraday yield.
The composition of this gaseous mixture depends on the type of cells used, on the nature of the electrodes used, and on the operating conditions of the electrolysis.
In cells with graphite anodes which have generally been used until the last few years, the composition of the mixture was as follows:
H.sub.2 : 91 to 95% PA1 O.sub.2 : 4 to 7% PA1 Cl.sub.2 : 0.4 to 0.8% PA1 CO.sub.2 : 0.4 to 1% PA1 H.sub.2 &gt;96% PA1 O.sub.2 &lt;3.5% PA1 Cl.sub.2 ; 0.2 to 0.5%
and the recovery and treatment of this gaseous effluent posed safety problems because of its flammable and explosive nature due to the relatively high oxygen content.
The solution of this problem generally adopted for transporting and treating this mixture in the required safe manner consisted in diluting the mixture in the cells or at the exit from said cells with an amount of air calculated so as to ensure that the hydrogen content of the mixture of electrolysis gas is less than 4%, which corresponds to at least 25-fold dilution of the electrolysis gas.
Since the 1970's a new cell technology has been developed using titanium anodes coated with an electro-active layer, making it possible, with the aid of new operating conditions, to achieve a yield exceeding that of cells using graphite anodes, and to obtain a gaseous mixture having the following composition:
which is outside the explosive range.
While it is possible to resort to the previous solution, it leads to excessively high dilutions which require the use of high-powered fans and involves a non-negligible energy consumption. Another major drawback of dilution is the fact that the recovery of the hydrogen, with a view to its subsequent utilization as a fuel or raw material, is virtually impossible.